1. Field of the Invention
The present invention relates to a display apparatus capable of restricting field radiation from a CRT (Cathode Ray Tube) thereof, a field radiation restriction circuit and a field radiation restriction electrode for restricting the field radiation.
2. Description of Related Art
A display apparatus having a CRT for use in a TV set, personal computer or the like involves radiation of an electric field to the environment of the display apparatus taking place mainly due to pulse voltage applied to a deflection yoke of the CRT. Since the field radiation is useless radiation, there arises a requirement for reducing the radiation toward the front portion of the image screen of the CRT in which a person viewing the image screen exists.
FIG. 1 is a schematic view showing the structure of a display apparatus having a structure similar to that disclosed in, for example, Japanese Patent Application Laid-Open No. 4-315741 and including a circuit capable of restricting field radiation from the CRT. Referring to FIG. 1, reference numeral 100 represents a display apparatus. The display apparatus 100 includes a CRT (Cathode Ray Tube) 1 having a tube surface 3, a deflection yoke 2 for scanning the image screen of the CRT 1, a cancellation electrode (a field radiation restriction electrode) 4 disposed adjacent to the tube surface 3 of the CRT 1 and capable of generating an electric field, a field radiation restriction circuit 99 for controlling generation of the electric field in the cancellation electrode 4, a horizontal deflection circuit 5 which is a circuit attached to the CRT 1 and capable of generating a horizontal-deflection retrace line pulse 6 to be applied to the deflection yoke 2, and a fly-back transformer 7 having a coil for transmitting a signal 8 having the polarity in reverse to that of the retrace line pulse 6. The field radiation restriction circuit 99 includes an amplitude adjuster 9, composed of two capacitors 10 and 11, and a phase adjuster 12 composed of a capacitor 10 and a variable resistor 13.
The horizontal deflection circuit 5 transmits the retrace line pulse 6 to the fly-back transformer 7. The fly-back transformer 7 transmits, to the field radiation restriction circuit 99, the output signal 8 having a waveform similar to that of the retrace line pulse 6 and having the polarity in reverse to that of the retrace line pulse 6. The field radiation restriction circuit 99 applies voltage to the cancellation electrode 4. The deflection yoke 2 radiates a radiated field 14 having a waveform 15 toward the front portion of the tube surface 3 of the CRT 1, while the cancellation electrode 4 radiates a cancellation field 16 having a waveform 17.
The operation will now be described. Mainly due to the horizontal-deflection retrace line pulse to be applied to the deflection yoke 2 during the operation of the CRT 1, the radiated field 14 is radiated from the tube surface 3 of the CRT 1. To cancel the radiated field 14, voltage is applied to the cancellation electrode 4, the voltage causing an electric field, having the polarity in reverse to that of the radiated field 14, to be generated. The voltage having the reversed polarity causes the cancellation electrode 4 to radiate the cancellation field 16. The waveform 17 is a reversed waveform like the waveform 15 of the radiated field 14 and synchronizes with the radiated field 14. Therefore, the radiated field 14 from the tube surface 3 of the CRT 1 is substantially canceled by the cancellation field 16.
As a voltage signal for causing the cancellation electrode 4 to generate the cancellation field 16 having the waveform 17, a signal for use in another purpose is employed. For example, the signal obtainable from the retrace line pulse 6 of the horizontal deflection circuit 5 has a waveform somewhat similar to the waveform of the radiated field 14 radiated from the deflection yoke 2 toward the front portion of the tube surface 3 of the CRT 1 and synchronized with the radiated field 14. Therefore, as the voltage signal to be supplied to the cancellation electrode 4, the output signal 8 obtainable from, for example, the fly-back transformer 7, is usually employed.
Referring to FIG. 1, the voltage obtained in response to the output signal 8 from the fly-back transformer 7, to which the retrace line pulse 6 is supplied from the horizontal deflection circuit 5, is applied to the cancellation electrode 4 through the phase adjuster 12, composed of the capacitor 10 and the variable resistor 13, and the amplitude adjuster 9 composed of the capacitors 10 and 11. By adjusting the phase adjuster 12 and the amplitude adjuster 9, the waveform of the output signal 8 from the fly-back transformer 7 is made to be similar to a waveform which is able to effectively cancel the radiated field 14 from the tube surface 3 of the CRT 1.
To describe a problem experienced with the conventional display apparatus, the waveform 15 of the radiated field 14 from the tube surface 3 of the CRT 1 and the waveform 17 of the cancellation field 16 from the cancellation electrode 4 are, in detail, shown in FIG. 2. Referring to FIG. 2, reference numeral 50 represents an under shoot portion of the waveform 15, and 51 represents a waveform which is the difference between the waveform 15 and the waveform 17. As shown in FIG. 2, the waveform 15 of the radiated field 14 is different from the waveform 17 of the cancellation field 16 at a point at which the under shoot portion 50 is formed as a waveform component. Therefore, the difference as indicated by the waveform 51 shown in FIG. 2 is unintentionally left. Thus, a satisfactory cancellation effect cannot be obtained. Even if the output signal 8 from the fly-back transformer 7 is divided by the capacitors 10 and 11 after allowed to pass through the variable resistor 13, the undershoot waveform component cannot be created. On the contrary, the phase of the waveform of the output signal 8 is delayed and, therefore, the waveform is made to be a further different waveform. As a result, the electric field cannot easily be canceled. As described above, the waveform 15 of the radiated field 14 from the tube surface 3 of the CRT 1 and the waveform 17 of the cancellation field 16 from the cancellation electrode 4 cannot easily be made to be completely similar to each other. Therefore, there arises a problem in that the radiated field 14 cannot satisfactorily be canceled.
In CRT display apparatus developed recently, the scanning frequency can be varied depending upon the type of the adapted personal computer. In a case where the scanning frequency has been switched, the waveform 15 of the radiated field 14, in particular, the size of the under shoot portion 50 is changed. Since the phase adjuster 12 and the amplitude adjuster 9, however, have constant frequency characteristics, the cancellation adjustment is disordered if the scanning frequency is changed. Although cancellation can be performed at a certain scanning frequency, cancellation of the electric field cannot satisfactorily be performed if the frequency is changed. Thus, the radiated electric field is unintentionally intensified.
Another conventional circuit capable of restricting the field radiation from the tube surface 3 of the CRT 1 has been disclosed in Japanese Patent Application Laid-Open No. 7-142008, wherein a cancellation electrode is disposed between an external graphite member of the CRT and an opening in the deflection yoke to cause the physical positions of the cancellation electrode and the deflection yoke to approach each other. However, a special fitting tool is required to dispose the cancellation electrode at the foregoing position, thus causing a problem to arise in that the working property deteriorates.
Another conventional structure has been disclosed in Japanese Patent Application Laid-Open No. 6-289801, wherein cancellation pulse voltage is superimposed on the output voltage from a high voltage generating circuit to cause the cancellation field to be radiated from a shadow mask (an aperture grille) through an anode. However, a special and costly voltage resisting capacitor is required to superimpose the cancellation voltage on the high voltage generating circuit.